Rust blueing is what I'd recommend on larger areas of a gun - not the small parts. This is because to get a good blue using rust blueing, you typically need to make several applications, then you let the rust grow in the right environment of humidity and heat, then you boil the red rust that forms in boiling water to turn it to black rust, then you "card off" the part with 0000 de-oiled steel wool or a very fine steel brush, then re-apply the rust blue chemical and repeat.
Rust blueing is used on high end guns, especially guns with solder work like custom hunting rifles with sweat-on sights, or double-barrel shotguns or rifles with soldered-on ribs. These types of guns should never be hot salt blued, because the salts attack the solder and (in the case of older double shotguns) get under the ribs and can't be flushed out. I've worked on double barrel shotguns where some yahoo had dunked the whole action/barrel setup into the hot salt blue tank and never boiled out the salts - there were salts in the action, which led me to start flushing the barrel ribs out. I could never get all the salts out by flushing, so I had to pull up the lower rib on the barrels and then remove all the salts, then polish out the barrels, sweat the rib back on and polish off the solder. Then, finally, I could re-blue the barrels.
By the time I was done, I was thinking that whoever pulled the stock off this nice old shotgun and just threw the entire, assembled action and barrels into the tanks should have been charged with criminal mischief. What a dismal mess.
For some small parts in some applications, I use nitre blueing - you get some potassium/sodium nitrate salts, you heat them up in stainless steel tank until they turn to liquid. Keep a thermometer in the tank of salts to make sure you keep the temps in the right range. You dip your parts into the tank for, oh, a minute to two minutes and they turn blue. I like using this on flat springs that I'm making because it both tempers the spring and blues it a nice color all at the same time. Sometimes, I'll use nitre blueing for small parts, especially screws, on a gun - but you can't do just "one or two" of the screws - you have to do all of them, because nitre blueing has a more blue hue (whereas most blueing is actually black). Done properly, it makes the screwheads on a high end gun "pop" as you look at it - but you have to take into account the overall effect here.
Here's a video by Larry Potterfield about nitre blueing:
And a Brownells article:
You could get a similar effect if you're really good with a propane torch and you have a nice, light touch on the heat - but most people don't, and they over-heat the part. This is why you want the thermometer in the nitre salts to keep the temperature at the 570 to 650F or so area. As steel heats up, it changes colors. When we gunsmiths are making chisels and screwdrivers, we talk about tempering the hardened tips of these tools back to a certain color - the lower temps (higher hardness) are the straw range, and purples and blues happen at higher temperature tempers (leaving the steel a bit softer) until finally you get into the pale blue/silver range, when you're taking quite a lot of hardness out in the tempering.
One of the things you can do to increase the effectiveness of cold blues is to clean the parts by dipping in acetone, let dry, then heat the parts before you dunk them in the solution. It doesn't have to be much - like to 140F. After you dunk parts in cold blue solutions, rinse them off, then dunk them in water displacing oil (like LPS-2, or WD-40) and let them sit there for an hour or so. Pull the parts out of the oil, put on a towel and don't handle them for a day.
You can keep using the cold blue solution you've got in some little container, but as it becomes more contaminated, there will come a point where you don't and won't get the results you want. At that point, might as well chuck it and get some more solution from the clean bottle.